R.G.J. Flay

Characteristics of the turbulence in the flow at a tidal stream power site

This paper analyses a set of velocity time histories which were obtained at a fixed point in the bottom boundary layer of a tidal stream, 5 m from the seabed, and where the mean flow reached 2.5 m s−1. Considering two complete tidal cycles near spring tide, the streamwise turbulence intensity during non-slack flow was found to be approximately 12–13%, varying slightly between flood and ebb tides. The ratio of the streamwise turbulence intensity to that of the transverse and vertical intensities is typically 1 : 0.75 : 0.56, respectively. Velocity autospectra computed near maximum flood tidal flow conditions exhibit an f−2/3 inertial subrange and conform reasonably well to atmospheric turbulence spectral models. Local isotropy is observed between the streamwise and transverse spectra at reduced frequencies of f>0.5. The streamwise integral time scales and length scales of turbulence at maximum flow are approximately 6 s and 11–14 m, respectively, and exhibit a relatively large degree of scatter. They are also typically much greater in magnitude than the transverse and vertical components. The findings are intended to increase the levels of confidence within the tidal energy industry of the characteristics of the higher frequency components of the onset flow, and subsequently lead to more realistic performance and loading predictions.

A new force balance data analysis method for wind response predictions of tall buildings

The theory currently used to predict the wind-induced response of buildings from force balance measurements is briefly reviewed. For buildings with coupled 3-D mode shapes, sources of uncertainty in the techniques response predictions are not just from errors in the mode shape corrections, but also from limitations in not being able to allow for coupled terms and higher mode effects. A new force balance data analysis technique which is designed to overcome these limitations is described. The new method eliminates the need to guess mode shape correction factors for buildings with non-linear sway and non-uniform torsional mode shapes. The reliability and accuracy of this method has been examined and validated analytically and experimentally to date. Some results of the analytical studies are presented here which demonstrate the power of the new method. It is believed that the new method will become the standard force balance data analysis method of tomorrow.

Integral length scales in strong winds below 20 m

Integral length scale measurements of the velocity components u, v and w for separations in the x, y and z directions obtained from Gill-type propeller anemometers are described. Scale measurements for longitudinal separations Lxi were made on a single 20-m-high tower and values obtained from both spectra and correlations. Scales for vertical separations Lzi were obtained from correlation measurements on the same tower. Scales for lateral separations Lyi were measured using an array of eight 10-m-high towers arranged in a straight line perpendicular to the wind direction under study. The spectral measurements give integral scales that are smaller than those obtained by integrating correlation functions. Using either a corrected correlation curve or assuming negative exponential behaviour and taking the time for the correlation to fall to a value of 1/e give results which are more consistent with the spectral results. The scales measured in the present study have values which are in agreement with the relatively few values given in the literature.

A twisted flow wind tunnel for testing yacht sails

Abstract The paper outlines the requirements for wind tunnel testing model yachts for sail aerodynamics investigations. It is shown that the apparent wind onto a yacht is “twisted” over the mast height, due to the vector addition of the yacht and wind velocities. The design features of a special wind tunnel built in New Zealand which can produce twisted flow in the test section which is suitable for testing model yachts are described. This wind tunnel is basically a conventional open-circuit long test-section boundary-layer wind tunnel, with the addition of some vertical vanes at the outlet. These can be twisted to desired orientations to achieve flow in the model test region which has the velocity and direction varying with height in the required manner. Some commissioning results from one twist configuration are presented which illustrate the success of the method. Some of the drawbacks of this approach which are difficult to overcome are also outlined.

A simulation model for wind turbine blade fatigue loads

The paper describes a horizontal axis wind turbine time domain simulation and fatigue estimation program written using the DelphiTM language. The program models the flapwise motion of a single rotor blade to determine the blade-root fatigue damage of a medium size wind turbine. The effects of turbulence intensity, mean wind speed, wind shear, vertical wind component, dynamic stall, stall hysteresis, and blade stiffness were examined. When all these effects were simulated it is found that a reduction in life of about 2 occurs between a low wind speed low turbulence intensity site, compared to a high wind speed high turbulence intensity site.

The economic optimisation of horizontal axis wind turbine design

Abstract A method for determining the optimum design parameters for horizontal axis wind turbines was developed and tested. These design parameters were the rotor diameter, rated power and tower height. The optimum values were found to be dependent on site wind regime. The results of the study indicated that it was, however, only the optimisation of the relative combination of rotor diameter and rated power with respect to site mean annual wind speed that afforded significant reductions in energy production cost. This optimisation confirmed that presently available wind turbines were optimised for mean annual wind speeds in the range 6–8 m/s, and suggested that for windier sites the energy production cost may be reduced by up to 10% through the optimisation of machine rated wind speed to suit such sites.

DEVELOPMENT OF A WIND TUNNEL TEST FACILITY FOR YACHT AERODYNAMIC STUDIES

Abstract Wind-tunnel tests on scale models of yachts are very difficult to carry out accurately because of the complicated aerodynamics and boundary conditions associated with the full-scale situation. Yachts sail at the interface between water and air. The water surface is not smooth, and the yacht orientation and wind strength vary with time. In addition, the sails behave somewhat like slender aerofoils for some sailing directions, and whereas like bluff bodies at others. This paper addresses some of the simplifying assumptions which must be made in order to make practical wind tunnel measurements on scale models of yachts. It describes a wind tunnel and balance facility which has been developed in the Mechanical Engineering Department of the University of Auckland to measure and process the aerodynamic forces and moments generated by wind flowing over yacht sails. The development of a linerised version of Galways force balance calibration method to process the balance data is described. This simplified version confers significant advantages in the real-time operation of the balance. Finally, the usefulness of the facility is illustrated with some results obtained during the study of a Twelve Metre Americas Cup Yacht sailing downwind under spinnaker.

Unsteady aerodynamics of two interacting yacht sails in two-dimensional potential flow

In this paper, unsteady thin aerofoil theory is extended to the case of two interacting aerofoils that oscillate harmonically perpendicular to the direction of the incident flow. The two aerofoils represent the headsail and mainsail of a yacht that sails upwind in waves. The developed theory is validated against theoretical data from the literature and results from wind tunnel tests with rigid, high-aspect ratio sail models oscillating at reduced frequencies from k = 0 to k = 0.68. Good agreement is found between the predicted and measured chordwise pressure distributions. An application of the theory to the case of an International Americas Cup Class yacht reveals that the time-varying components of the aerodynamic forces are small and that the thrust gain is minimal, i.e. only very little energy can be extracted from the unsteady flow about the sails. No attempt is made to investigate the influence of the flexibility of the sails, three-dimensional effects or phenomena related to dynamic stall.

Compliant blades for passive power control of wind turbines

Recent advances in analysis capabilities for wind turbine blade aerodynamics and structural dynamics enables a thorough understanding of the connection between the aerodynamic excitation and structural response of blades. By designing the blades using fibre reinforced composite materials, coupling between bending and torsion, for example, can be built-in. In the present work the objective was to investigate the capability of a 50 kW constant speed wind turbine to automatically shed power in gusts by feathering the blades, i.e. twisting them towards the relative wind vector thus reducing the angle of attack, whilst bending away from the wind. This numerical study demonstrated that it is difficult to achieve constant power output with compliant blades for a fixed speed wind turbine because a large amount of twist is required. This paper details the investigation and results obtained in this aeroelastic study of compliant blades made from fibre reinforced composite materials.

Pressure correlations on a rough cone in turbulent shear flow

Abstract A detailed wind tunnel test has been carried out on a rough cone model using an electronically scanned pressure transducer system, enabling almost instantaneous capture of 256 pressure tap signals from 16 levels of 16 equispaced taps at each level. A very complete analysis was performed on a reduced data set consisting of weighted integrations of the 16 pressure signals forming time series of lift and drag at each level. Autospectra of the drag data showed good agreement with the approach wind spectra at the same level, and with the von Karman spectral equation. Local aerodynamic admittance showed less reduction with increasing fD/V than expected from “lattice plate” theory. Drag root-coherence could be well described by a modified exponential decay equation, and differed considerably from the root-coherence of the longitudinal component of the onset wind flow. The lift autospectra were well fitted by the forced-damped-harmonic motion vibration equation. Normalised co-spectra were obtained between reference levels 5, 9, and 12 and each of the other 15 levels and were used to find correlation coefficients at the shedding frequency of the reference level. The coefficients collapsed to a single curve when plotted versus non-dimensional separation, which could be well fitted by an exponentially damped cosine function. Analysis of the 16 front and 16 rear taps showed that the root-coherence of the front pressures was more similar to the wind coherence than the drag pressures, but the root-coherence of the rear tap pressures was substantially less than the wind for low dimensionless frequencies.